Figure 1 above shows a test setup in which an ESD current passes through
two adjacent current probes positioned on the current carrying wire. A
floating
metal plate, held up by a book, is placed near but not
contacting the wire. Figures 3 and 4 show the resulting output of
the two current probes. The upper trace is from the current probe closest
to the ESD simulator and the lower trace is the downstream probe (the one
on the left above). The outputs are essentially identical except that the
downstream probe shows slightly less high frequency detail (slightly longer
risetime).

When the otherwise floating plate is making
contact with the wire, the resultant current probe outputs are shown
in Figures 5 and 6 below. Notice that the risetime of the downstream probe
is significantly lengthened. This results from a current on the wire after
the plate that has much lower di/dt and therefore reduced interference
potential.

The free space capacitance of the plate represents a lower impedance
to the high frequency components of the ESD current than the inductance
of the wire. Therefore, the high frequency components of the current enter
the plate and do not show up in the output of the downstream current probe.
A metal plane used for filtering high frequency currents this way is often
referred to as a Transient Suppression Plane.

Why does the output of the downstream probe in Figure 4 have a slightly
longer risetime than the upstream probe? Click here
for the answer. The effect is similar to the effect of the plate above.

Figure 3. Current Probe Outputs
(Plate Not Connected)

Figure 4. Expanded View of Figure 3
(Rising Edge)

Figure 5. Current Probe Outputs
(Plate Touching Wire)

Figure 6. Expanded View of Figure 5
(Rising Edge)

A pair of current probes with a typical transfer impedance accuracy
of +/- 2 dB will likely have transfer impedances that are not close enough
for comparing the outputs on a linear scale as has been done here. The
data in Figures 3-6 was taken with a pair of Fischer F-33-1 current probes
that have transfer impedances matched to within about 2%.